Data communication systems are widely-known in the art. These systems enable heterogeneous computers to communicate with each other using a defined set of rules and message exchanges, known as data communication protocols. Data communication protocols are structured based on the concept of protocol layering. For instance, the data communication functions are partitioned into a hierarchical set of layers where each layer performs a related subset of the functions required to communicate with another system. Each layer relies on the next lower layer to perform more primitive functions and to conceal the details of those functions. Each layer also provides services to the next higher layer. Of course, it takes two to communicate, so the same set of layered functions must exist in two systems. Communication is achieved by having the corresponding or peer layers in two systems communicate using predefined protocols. For example, a well-known framework for defining standard data communication protocols is the Open Systems Interconnection (OSI) reference model, which was established by the International Organization for Standardization. In the OSI architecture, each system communicating with another system contains seven protocol layers: physical layer, data link layer, network layer, transport layer, session layer, presentation layer, and application layer.
A well-known suite of protocols used in many communications systems is based on asynchronous transfer mode (ATM). ATM is a well-known cell-oriented switching and multiplexing data communication technology that utilizes fixed-length packets or cells to carry different types of traffic. Each cell is 53 bytes in length and comprises a 5-byte header and a 48-byte payload. Each cell is switched and multiplexed throughout the ATM network based on the information contained in the header. The cell header identifies the destination of the cell, the cell type, and the cell priority. For example, the header comprises a virtual path identifier (VPI) field and a virtual channel identifier (VCI) field, which have local significance only and identify the destination of the cell. The header also comprises a generic flow control (GFC) field, which allows a multiplexer to control the rate of an ATM terminal. The header further comprises a payload type (PT) field, which indicates whether the cell contains user data, signaling data, or maintenance information and a cell loss priority (CLP) field, which indicates the relative priority of the cell. Using the CLP field, lower priority cells are discarded before higher priority cells during congested intervals. The header also comprises a cell header error check (HEC) field, which detects and corrects errors in the header. The payload field is passed through the network intact, with no error checking or correction. ATM relies on higher layer protocols to perform error checking and correction on the payload.
When using ATM, longer packets cannot delay shorter packets as in other packet switched implementations because long packets are divided into many fixed-length cells. This enables ATM to carry constant bit rate (CBR) traffic, such as voice and video, in conjunction with variable bit rate (VBR) data traffic, potentially having very long packets in the same network.
The two lowest protocol layers in the ATM protocol stack are the physical (PHY) layer and the ATM layer. The PHY layer provides for transmission of ATM cells over a physical medium that connects two ATM devices. The bits in the cells are transmitted over the transmission medium in a continuous stream. All information is switched and multiplexed in the ATM network in these fixed-length cells.
In ATM communication systems, the ATM layer provides the switching and multiplexing of virtual path connections (VPC) and virtual channel connections (VCC) between systems. Systems and methods for providing communication between an ATM layer device and multiple PHY layer devices are known in the art. For example, the Universal Test & Operations PHY Interface for ATM (UTOPIA) level 2 specification defines a standard data path interface between an ATM layer device and multiple PHY layer devices in an ATM communication system for communicating data in order to effectuate ATM network switching. The details of UTOPIA may be found in the ATM Forum Technical Committee document entitled “UTOPIA Level 2, Version 1.0 (af-phy-0039-00), which is entirely incorporated herein by reference.
The UTOPIA bus was originally conceived for use in ATM switching nodes within the ATM network where the total number of ports (PHY layer devices) is typically fairly small. Thus, the UTOPIA bus was designed with a five bit addressing scheme. Thus, the total number of PHY layer devices that can be connected to the standard UTOPIA bus is thirty-one, with one invalid address used in the polling discipline to indicate there is no address or no poll.
It is also known in the art to provide ATM communications via digital subscriber line (DSL) technologies. DSL technologies have become a widely-used solution for providing high bit rate transmission over the existing copper wire infrastructure, known as the “subscriber loop.” DSL technologies dramatically improve the bandwidth of the existing analog telephone system. DSL enhances the data capacity of the existing copper wire that runs between the local telephone company switching offices and most homes and offices. The bandwidth of the wire has conventionally been limited to approximately 3,000 Hz due to its primary use as a voice telephone system. While the wire itself can handle higher frequencies, the telephone switching equipment is designed to cut-off signals above 4,000 Hz to filter noise off the voice line. DSL enables high-speed data traffic from a service provider network, such as an ATM network, to be provided on the existing wires with voice traffic.
In order to provide DSL service, a digital subscriber line access multiplexer (DSLAM) is employed at the local telephone company central office or digital loop carrier (DLC). The DSLAM includes frequency band filters to separate the voice-frequency traffic provided by the public-switched telephone network (PSTN) from the high-speed data traffic service provided by the network service provider. A DSLAM multiplexes the high-speed data traffic and routes it to subscribers on twisted-pair wires, referred to as a local loop. Many DSLAMs are designed to work with ATM networks.
Typically, a DSLAM includes an uplink interface, a switch concentration module (SCM), a backplane interface, and multiple line cards. High-speed data traffic from an ATM network is received by the uplink interface via multiple data communications channels. The high-speed data traffic is then transmitted to the SCM where it is transmitted to the backplane interface. The backplane interface provides the high-speed data traffic to multiple DSL ports in the line cards for subsequent delivery to subscribers.
One known type of DSL-based service is asymmetrical DSL (ADSL). ADSL is the most common DSL service. It is an asymmetrical technology, meaning that the downstream data rate is much higher than the upstream rate. The term upstream refers to data transfer toward the interior of the communication network. The term downstream refers to data transfer away from the interior of the communication network. In the context of a DSLAM and referring to the interface between the ATM layer device and DSL physical layer devices, the downstream direction corresponds to the transfer of cells from the ATM layer device to the physical layer devices for transmission over the DSL. The upstream direction corresponds to the transfer of cells received via the DSL from the physical layer devices to the ATM layer device. This type of service works well for providing typical Internet services to residential subscribers. ADSL operates in a frequency range that is above the frequency range of voice services, so the two systems can operate over the same subscriber cable.
For example, the ADSL standard of the International Telecommunications Union entitled “Recommendation G.992.1: Asymmetric Digital Subscriber Line (ADSL) Transceivers,” which is entirely incorporated herein by reference, proscribes two types of channels to be carried simultaneously over the subscriber loop. One type of channel is characterized by a reduced error rate. This type of channel, however, does incur considerable delay because the forward error correction technique incorporates an interleaver. The other type of channel does not use the interleaver and thus has lower delay and a potentially higher error rate. The low-delay channel is considered more suitable for transporting real-time circuits, such as those carrying voice or real-time video, because real-time circuits are willing to accept some transmission errors in order to reduce delays. On the other hand, non-real-time circuits, such as those carrying data, are comparatively intolerant of errors because any single error requires retransmission of the entire block. Furthermore, circuits carrying data are not adversely effected by longer delays. Therefore, the low error rate channel is well-suited for carrying data circuits.
It may also be desirable to provide separate access means for real-time and non-real-time data paths in a variety of other situations. For example, if the data transmission technology employed in the physical layer device requires substantial local buffering of data, such as for half-duplex transmission, separate access for the real-time data may be necessary to prevent the presence of lower priority data in the internal buffer from blocking the immediate transmission of high-priority data. In this case, the separate access means for the real-time (priority) data provides a way to effectively bypass already buffered lower priority data.
When using both ADSL channels, the entire bandwidth available for payload data over the DSL must be statically partitioned between the low-delay channel and the high-reliability channel. From the point of view of the ATM layer device, these channels are independent circuits and proper management of traffic over the circuits requires that the ATM layer device provide a scheduling function connected to each channel. The only way to satisfy this requirement with “off the shelf” ATM layer devices and UTOPIA interfaces is to provide a separate UTOPIA port for each of the two channels. Thus, in the context of ADSL, each PHY layer device requires two separate UTOPIA bus addresses, one for the low-delay channel and one for the high-reliability channel.
FIG. 1 illustrates a known system for providing communication between an ATM layer device and multiple dual-channel PHY layer devices via a local interface, such as a UTOPIA bus. The ATM layer device supports a predefined number (N) of virtual channels. Each PHY layer device comprises two channel ports corresponding to two different types of channels. As shown in FIG. 1, each virtual channel communicates with one of the channel ports in one of the PHY layer devices via a separate address corresponding to the local interface. Thus, because each PHY layer device supports two types of channels, the ATM communication system that supports N virtual channels on the ATM layer device and N addresses on the local interface is restricted to (N/2) PHY layer devices.
The UTOPIA bus was originally conceived for use in ATM switching nodes within the network where the total number of ports connected to a switch is typically fairly small. Thus, as described above, the total number of PHY layer devices that can be connected to the standard UTOPIA bus is thirty-one (one invalid address). However, in systems such as those described above where more than one type of channel is supported by each PHY layer device, the total number of PHY layer devices that may be used with the UTOPIA address is substantially reduced. For instance, where two types of channels are employed, the UTOPIA bus can only support half as many, for instance, fifteen in the example above, dual-channel PHY layer devices.
The reduction in the number of PHY layer devices is very problematic. For example, in the DSL environment where many subscribers are served by a single ATM switching node, such as a DSLAM, it is advantageous to be able to connect a very large number of PHY layer devices to a single ATM layer device.
One known solution to this problem proposes including multiple ATM layer devices in the communication system. This approach, however, is also problematic. For instance, including multiple ATM layer devices significantly increases the complexity, cost, and power consumption of the communication system. Furthermore, where the communication system also includes a DSLAM, including multiple ATM layer devices also increases the complexity, cost, and power consumption of the ATM layer device in the DSLAM and may require modification to the DSLAM backplane. In addition, the inclusion of additional ATM layer devices may actually require so much space as to preclude achieving the desired ratio of PHY layer devices. Furthermore, other solutions all by necessity use a non-standard technique to expand the address space. This limits the choices for physical and ATM layer devices, and, in so doing, defeats the purpose of a standard interface, such as the UTOPIA bus, which is to expand the range of candidate devices for building ATM systems.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.